With sufficient data and the appropriate modelling tools, it is possible to replicate the real-life behaviour of geothermal systems. Modelling tools could guide geologists, engineers and decision-makers in developing an optimal design for these systems. In order to explore the effectiveness of the modelling techniques, this paper presents a new three-dimensional (3D) numerical model of a hot dry rock (HDR) geothermal reservoir using coupled thermo-hydro-mechanical (THM) processes. The model is implemented in the COMSOL Multiphysics Finite Element (FE) solver, and its reliability is confirmed by conducting a validation study using field measurements from the Fenton Hill Phase I HDR system in New Mexico. After confirming the model's reliability, two case studies are analysed to determine the different factors affecting reservoir performance during exploitation. The factors analysed for the two cases are wellbore alignment and placement, using single and multiple planar fracture HDR system configurations. The results show that wellbore alignment does not affect changes in fracture properties and reservoir productivity. Wellbore placement, however, is found to affect reservoir performance significantly. The simulation analysis performed shows that knowledge of injection/production wellbore placement could be a significant asset to reservoir engineers/managers during the planning, exploration, design, and exploitation stages.